Composition and method for polishing a composite of silica and silicon nitride

ABSTRACT

A composition is provided for polishing a composite comprised of silica and silicon nitride comprising: an aqueous medium, abrasive particles, a surfactant, and a compound which complexes with the silica and silicon nitride wherein the complexing agent has has two or more functional groups each having a dissociable proton, the functional groups being the same or different.

This application claims the benefit of U.S. Provisional application Ser.No. 60/027,277 filed Sep. 27, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compositions which are useful asslurries for the chemical-mechanical polishing of substrates, especiallysubstrates comprised of silica and silicon nitride. More specificallythe slurries of the present invention include an aqueous medium,abrasive particles, a surfactant, and a compound which complexes withsilica and silicon nitride.

2. Description of Related Art

In integrated circuit technology, various active and passive elementsgenerally must be isolated from one another within the integratedcircuit structure. This has often been accomplished by deep or shallowtrench isolation techniques. These techniques typically use silicondioxide (silica) as a dielectric material and silicon nitride as a stoplayer, requiring chemical-mechanical polishing (planarization) of eachcircuit layer. To achieve efficient polishing and adequateplanarization, a polishing slurry is generally useful and should providea high selectivity involving the removal rate of silica relative tosilicon nitride.

In Silvestri et al., U.S. Pat. No. 4,526,631, a slurry of 6 weightpercent colloidal silica adjusted to a pH of about 12 with KOH providesa polishing ratio of about 10 SiO₂ to 1 Si₃ N₄. Beyer et al., U.S. Pat.No. 4,671,851, states that the polishing ratios between SiO₂ and Si₃ N₄preferably should be between a lower limit of 4 to 1 and a higher limitof 40 to 1. Beyer describes obtaining a ratio of 6.2 to 1 using acolloidal silica in water with small amounts of a sodium salt ofdichloroisocyanuric acid and sodium carbonate.

Even more recent patents such as Murase, U.S. Pat. No. 5,502,007, alsodescribe obtaining selectivities of about 10 SiO₂ to 1 Si₃ N₄ removalrates using a colloidal silica slurry as a polishing agent. Kodera etal., U.S. Pat. No. 5,445,996, use ceria as well as silica for theabrasive particles in slurries, but they also report selectivities forSiO₂ to Si₃ N₄ removal rates in the range of 2 to 3.

SUMMARY OF THE INVENTION

A composition is provided for polishing a composite comprised of silicaand silicon nitride comprising: an aqueous medium, abrasive particles, asurfactant and a compound which complexes with the silica and siliconnitride wherein the complexing agent has two or more functional groupseach having a dissociable proton, the functional groups being the sameor different.

A further aspect of the invention is the method for polishing acomposite comprised of silica and silicon nitride comprising: applying aslurry at a polishing interface between a polishing pad and thecomposite comprised of silica and silicon nitride, the slurrycomprising: an aqueous medium, abrasive particles, a surfactant and acompound which complexes with the silica and silicon nitride wherein thecomplexing agent has two or more functional groups each having adissociable proton, the functional groups being the same or different.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that the addition of a compound which complexes withsilica and silicon nitride to polishing slurries used in thechemical-mechanical polishing of composites comprised of silica andsilicon nitride can provide a very high selectivity of rate of removalof SiO₂ to the rate of removal of Si₃ N₄ when a surfactant is used inconjunction with the complexing agent and when the concentration of thecomplexing agent in the slurry is sufficient to block the removal of Si₃N₄ while not greatly affecting the removal of SiO₂ at the pH of thepolishing slurry.

Compounds which act as complexing agents or chelating agents for SiO₂and Si₃ N₄ are described in great detail in U.S. Pat. No. 5,391,258 andU.S. Pat. No. 5,476,606 which are made part of this specification byreference. These compounds must have at least two acid groups present inthe structure which can affect complexation to the silica and siliconnitride surface. Acid species are defined as those functional groupshaving a dissociable proton. These include, but are not limited to,carboxylate, hydroxyl, sulfonic and phosphonic groups. Carboxylate andhydroxyl groups are preferred as these are present in the widest varietyof effective species. Particularly effective are structures whichpossess two or more carboxylate groups with hydroxyl groups in an alphaposition, such as straight chain mono- and di-carboxylic acids and saltsincluding, for example, malic acid and malates, tartaric acid andtartarates and gluconic acid and gluconates. Also effective are tri- andpolycarboxylic acids and salts with secondary or tertiary hydroxylgroups in an alpha position relative to a carboxylic group such ascitric acid and citrates. Also effective are compounds containing abenzene ring such as ortho di- and polyhyclroxybenzoic acids and acidsalts, phthalic acid and acid salts, pyrocatecol, pyrogallol, gallicacid and gallates and tannic acid and tannates. In the examples tofollow, a salt of phthalic acid is used as the complexing agent and,therefore, such salts are preferred complexing agents for thisinvention. Potassium hydrogen phthalate, "KHP", was the phthalate saltused in the experiments described below.

The surfactant used in conjunction with the complexing agent in thisinvention is not present to perform the usual function of surfactants inslurries of stabilizing the particulate dispersion. As shown in theexamples which follow, the surfactant in combination with the complexingagent affects the rate of removal of Si₃ N₄ from the composite surface.It is believed that any surfactant, whether it be an anionic, cationic,non-ionic or zwitter-ionic surfactant, might be effective in thecompositions of this invention. Particularly useful would befluorocarbons or hydrocarbons with phosphate end groups. In thefollowing examples several different surfactants were shown to beeffective. "ZFSP", ZONYL™ FSP Fluorosurfactant, manufactured by theDuPont Company, was shown to be a particularly effective surfactantadditive to the slurries of this invention. It is a long straight chainhydrocarbon with phosphate groups at one end and a fluoride at the otherend.

In these examples, ceria was used for the abrasive particles in theslurry because it is an effective polishing abrasive forchemical-mechanical polishing at all pH conditions and is stable againstgelation. Any other polishing abrasive, such as alumina, zirconia,silica, titania and barium carbonate could also be used.

To adjust the slurries of this invention to the pH range in which thehighest selectivities of SiO₂ removal to Si₃ N₄ removal are obtained anybase or amine compound might be used. In the examples to follow KOH isused to adjust the pH of the slurry compositions. Potassium hydroxide,ammonium hydroxide, and all types of soluble amine compounds may be usedto adjust the pH of chemical-mechanical polishing slurries.

EXAMPLE 1

Table 1 shows the results of polishing silicon dioxide and siliconnitride wafers with slurries containing various amounts of complexingagent at selected pH levels. These experiments were carried out on aStrasbaugh 6DS SP Planarizer using an IC 1000/SubaIV polishing pad stackunder the conditions of 7 psi down pressure, 1.5 psi back pressure, 30rpm carrier speed and 32 rpm table speed with a slurry flow rate of 125ml/min. 6 inch wafers were used and the pad was conditioned after eachwafer was polished. All slurries in this series of experiments contained0.45% colloidal ceria and 0.2% ZFSP surfactant and the pH of the slurrywas adjusted using potassium hydroxide.

                  TABLE 1                                                         ______________________________________                                        Sample % KHP*    pH     RR SiO.sub.2                                                                         RR Si.sub.3 N.sub.4                                                                   Selectivity                            ______________________________________                                        1      0         4      1419   256     5.5                                    2      3.1       4      6      3       2                                      3      0.5       7      3019   189     16                                     4      1         7      3000   15      200                                    5      3.1       7      1185   4       296                                    6      1         10     3397   994     3.5                                    7      2         10     3246   920     3.5                                    ______________________________________                                         *Potassium hydrogen phthalate                                            

These results clearly show that selectivities of silicon dioxide tosilicon nitride removal rates much higher than reported heretofore canbe obtained by using slurries comprising a complexing agent and asurfactant at a pH level where these additives effectively shut down theremoval rate of silicon nitride while not greatly affecting the removalrate of silicon dioxide. As shown above, selectivities of 200 andgreater can be obtained by this inventive method of polishing.

EXAMPLE 2

The following experiments show the necessity of using a surfactant inthe slurries of this invention. They were carried out on a Strasbaugh6DS SP Planarizer under the same conditions described in Example I.Slurries were similar to the one used for Sample 4 on Table I in thatthe slurries were comprised of 1%KHP, 0.45% colloidal ceria and the pHwas adjusted to 7 using KOH. Percent surfactant, ZFSP, was either 0.2%as in Sample 4 or 0.0%. Removal rates for Si₃ N₄ in Angstroms per minuteare shown in Table 2 below:

                  TABLE 2                                                         ______________________________________                                                % ZFSP                                                                              RR Si.sub.3 N.sub.4                                             ______________________________________                                                0     793                                                                     0     768                                                                     0     736                                                                     0.2   10                                                                      0.2   8                                                               ______________________________________                                    

It is obvious from these data that the surfactant is critical to theshutting down of the Si₃ N₄ removal rate so that such extraordinaryselectivities can be obtained.

EXAMPLE 3

In the following experiments the abrasive in the slurries was commercialopaline ceria which was milled before use. Polishing of wafers wasaccomplished under the same conditions as in Example 1. Results aregiven in Table 3 below:

                  TABLE 3                                                         ______________________________________                                              %                           RR   RR                                     Sample                                                                              Abrasive % KHP    % ZFSP                                                                              pH  SiO.sub.2                                                                          Si.sub.3 N.sub.4                                                                   Selectivity                       ______________________________________                                        8     2        0        0     7   5804 3504 2                                 9     2        1        0.2   7   2642 22   120                               10    3.5      1        0.2   6.5 3195 13   246                               11    5        1        0.2   6.5 3705 33   112                               12    3.5      2        0.4   7   2133 10   212                               13    5        2        0.4   7.5 3609 1105 3                                 ______________________________________                                    

These results show that a complexing agent and a surfactant effectivelyshut down the removal rate of Si₃ N₄ when used in a slurry adjusted to adesired pH range. At a pH of 6.5 and 7 the removal rate of Si₃ N₄ isgreatly shut down giving selectivities for the removal of silica overthat of silicon nitride of over 100 (samples 9-12). However, at a pH of7.5 (sample 13), the removal rate of silicon nitride is no longerdepressed and selectivities are very poor.

EXAMPLE 4

In these experiments several surfactants were found to be effective inreducing the removal rate of silicon nitride at a pH of 6.5. Thesurfactants used were FC-93, Fluorad™ FC-93, an anionic fluorochemicalsurfactant available from the 3M Company; "PVS", the sodium salt ofpolyvinyl sulphonate available commercially; and "ZFSN", ZONYL™ FSN, anon-ionic surfactant available from the DuPont Company. The slurries inthis Example all contained 1.5% KHP (potassium hydrogen phthalate) and0.45% commercial opalene ceria as the abrasive. Polishing of wafers wasaccomplished under the same conditions as in Example 1. Results aregiven in Table 4 below:

                  TABLE 4                                                         ______________________________________                                        Sample Surfactant                                                                             % Surfactant                                                                            RR SiO.sub.2                                                                         RR Si.sub.3 N.sub.4                                                                  Selectivity                           ______________________________________                                        14     FC-93    0.2       2975   464    6                                     15     PVS      0.3       3406   35     98                                    16     ZFSN     0.3       2678   39     68                                    ______________________________________                                    

These results show that several surfactants are effective in slurriesmade to reduce the removal rate of silicon nitride. A given surfactantmight be even more effective should pH and slurry compositions beoptimized.

EXAMPLE 5

In this Example the abrasive used was WS2000 available from Rodel, Inc.WS2000 is an abrasive which contains both ceria and silica. The slurryused for this experiment contained 3.5% abrasive, 1.5% KHP (potassiumhydrogen phthalate), and 0.2% ZFSP (ZONYL™ FSP). The pH was about 6.5.Results from the polishing of wafers under the same conditions as inExample 1 are shown on Table 5 below:

                  TABLE 5                                                         ______________________________________                                        Sample   RR SiO.sub.2 RR Si.sub.3 N.sub.4                                                                    Selectivity                                    ______________________________________                                        17       2209         9        244                                            ______________________________________                                    

The above Examples show many embodiments of this invention and are notmeant to restrict the invention in any way. The scope of the inventionis defined only by the claims which follow:

We claim:
 1. A method of polishing a composite comprised of silica andsilicon nitride comprising:applying a slurry at a polishing interfacebetween a polishing pad and said composite comprised of silica andsilicon nitride, said slurry comprising: an aqueous medium, abrasiveparticles, a surfactant, and a compound which complexes with said silicaand silicon nitride, wherein said compound has two or more functionalgroups each having a dissociable proton, the functional groups being thesame or different.
 2. A method according to claim 1 wherein saidcompound which complexes with silica and silicon nitride contains abenzene ring.
 3. A method according to claim 1 wherein said compoundwhich complexes with silica and silicon nitride is a straight chainmono- or di-carboxylic acid or salt which has a secondary hydroxyl groupin an alpha position relative to a carboxylate group.
 4. A methodaccording to claim 1 wherein said compound which complexes with silicaand silicon nitride is a tri- or poly-carboxylic acid or salt which hasa secondary or tertiary hydroxyl group in an alpha position relative toa carboxylic group.
 5. A method according to claim 1 wherein saidabrasive particles comprise ceria.
 6. A method according to claim 1wherein said surfactant comprises a fluorosurfactant.
 7. A methodaccording to claim 2 wherein said compound which complexes with silicaand silicon nitride is potassium hydrogen phthalate.
 8. A methodaccording to claim 2 wherein said polishing composition comprises:water, about 0.2% to about 5% ceria, about 0.5% to about 3.5% potassiumhydrogen phthalate, about 0.1% to about 0.5% fluorosurfactant, allpercentages by weight, and wherein the pH of said polishing compositionis adjusted from about 6 to about 7 by the addition of a base or anamine compound to said polishing composition.